Preheat oven to 350 degrees F.

Sift together flour, cocoa, baking soda, and salt in a bowl and set aside.

In the bowl of an electric mixer, combine mayonnaise and sugar and beat at medium speed until blended.

Gradually add the cold water and vanilla while the mixer is running.

Add the dry ingredients to the mayonnaise mixture and beat until well blended, about 2 minutes.

Pour batter into a buttered 13x9x2-inch baking pan.

Bake in 350 degree F oven for 40 minutes or until a cake tester comes out clean.

Cool the cake in the pan on a rack.

In a saucepan over medium heat, combine brown sugar, half and half or light cream, and butter for the frosting.

Cook over medium heat, stirring constantly, until the mixture comes to a boil.

Let it boil for 1 minute, then remove from heat.

Stir in the chocolate chips and vanilla.

Beat with an electric mixer at high speed until the consistency is fluffy and spreadable.

When the cake has cooled, frost it with the Fluffy Chocolate Frosting.

Cut into squares and serve.